Abstract
This article describes the improvement of wastewater treatment in a fertilizer plant located in Central Italy (municipality of Vasto). In this facility, water is used for the removal of dust and fluorinated gases from the air. The resulting wastewater contains fluorides and phosphates in hazardous forms. Its treatment ordinarily does not result in a Zero-Liquid Discharge (ZLD) process. To achieve this purpose, several reagents were tested, focusing on the correlation linking pH, type of reagent and the effect on the separation of fluorides and phosphates from the wastewater. It was eventually found, and explained with a model, that hydrated lime at pH = 12 was so effective as a precipitating agent that phosphate and fluoride separation reached a value of 99.9%, thus allowing for reuse of the water in the plant process. Furthermore, phosphates and fluorides precipitated in a non-hazardous form, so that the material could also be recycled. In synthesis, wastewater treatment of the fertilizer plant was upgraded so that it became a ZLD process coupled with the recovery and recycling of fluorides and phosphates.
Highlights
The raw materials for the production of phosphate fertilizers, phosphoric acid, and elemental phosphorus are natural phosphate ores like apatite and phosphorite
In the manufacturing process of phosphate fertilizers, raw material containing phosphorus in the form of Ca3(PO4)2, which is an insoluble and poorly available form for plants, is chemically treated by sulfuric acid to obtain a product with a high concentration of P2O5, which can be absorbed by plants
After the characterization of two different wastewater samples, the effect of the flocculant was reported, the results obtained after the addition of NaCl and KOH or NaOH were discussed, together with the analysis of the chemistry and the precipitation mechanisms of fluorides and phosphates caused by lime
Summary
The raw materials for the production of phosphate fertilizers, phosphoric acid, and elemental phosphorus are natural phosphate ores like apatite and phosphorite. During acidulation of phosphate rock to produce phosphoric acid or superphosphates, fluoride is released as hydrofluoric acid HF, which in turn reacts with silica forming the volatile gas silicon tetrafluoride SiF4 and hydro-soluble hexafluorosilicic acid H2SiF6. Both of these are partially carried along in the wastewaters coming from the operations of gaseous waste scrubbing and phosphogypsum transportation performed during the processing of phosphate rock [1]. Wastewater from the industrial production of phosphate fertilizers is generated in a relatively small quantity It contains very fine suspended solids and a high concentration of hazardous substances (phosphoric acid H3PO4, hydrofluoric acid HF, and hexafluorosilicic acid H2SiF6) which make purification treatments difficult. The fertilizer industry discharges wastewater into the environment, including several thousand tons of fluorides per year and even more phosphates
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